Ground-based near-infrared emission spectroscopy of HD 189733B

We investigate the K- and L-band dayside emission of the hot-Jupiter HD 189733b with three nights of secondary eclipse data obtained with the SpeX instrument on the NASA Infrared Telescope Facility. The observations for each of these three nights use equivalent instrument settings and the data from one of the nights have previously been reported by Swain et al. We describe an improved data analysis method that, in conjunction with the multi-night data set, allows increased spectral resolution (R ∼ 175) leading to high-confidence identification of spectral features. We confirm the previously reported strong emission at ∼3.3 μm and, by assuming a 5% vibrational temperature excess for methane, we show that non-LTE emission from the methane ν₃ branch is a physically plausible source of this emission. We consider two possible energy sources that could power non-LTE emission and additional modeling is needed to obtain a detailed understanding of the physics of the emission mechanism. The validity of the data analysis method and the presence of strong 3.3 μm emission are independently confirmed by simultaneous, long-slit, L-band spectroscopy of HD 189733b and a comparison star.